Mixture, use of this mixture, and method for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy

ABSTRACT

The invention relates to a mixture to be introduced into the slag located on a metal melt in iron and steel metallurgy, the use of such a mixture, and a method for conditioning a slag located on a metal melt in a metallurgical vessel, for example in a converter, in an electric arc furnace or in a ladle, in iron and steel metallurgy.

The invention relates to a mixture to be introduced into the slag located on a metal melt in iron and steel metallurgy, the use of such a mixture, and a method for conditioning a slag located on a metal melt in a metallurgical vessel, for example in a converter, in an electric arc furnace, or in a ladle, in iron and steel metallurgy.

In steel and iron metallurgy the pig iron melt is separated from undesired constituents before casting.

If a converter is used then in the case of the LD method, which is currently the most widespread, oxygen is blown for this purpose by means of a lance onto the pig iron melt located in a converter lined with an alkaline refractory material. The process of this blowing of oxygen onto the pig iron melt is also referred to as refining. During refining, iron contaminants, in particular iron contaminants in the form of carbon, manganese, silicon and phosphorous, are oxidised by the oxygen blown in and together with added burnt lime form a slag layer floating on the metal melt.

In an electric arc furnace the crude steel melt is produced by melting scrap metal, pig iron, liquid iron and/or direct reduced iron and other raw materials.

Once the metal melt refined in the primary metallurgical apparatus has the desired properties, this is tapped through the tapping channel into the ladle for secondary metallurgical treatment.

The slag must be selectively influenced or conditioned with regard to chemical and physical properties.

In order to condition the slag it is known to provide the slag with what are known as slag conditioners in order to be able to change the properties of the slag.

The basicity, i.e. the mass or molar ratio of the alkaline components to the other components of the slag (which for example can be calculated in accordance with the following formula: [xCaO+MgO]/[xSiO2+Al₂O₃+further components]), of the initially acidic or non-alkaline slag must therefore be increased in order to reduce the corrosive attack of the slag on the alkaline lining of the metallurgical vessel in which the metal melt is located and thus attenuate the erosion of the lining and increase the service life thereof. For this reason, slag conditioners have a component increasing the basicity of the slag, this component in particular being lime, dolomite lime, or dolomite.

It is additionally expedient to adjust the content of MgO in the slag by addition of a slag conditioner, such that it lies in a range of saturation of MgO in the slag and a corrosive attack of the slag on the lining is thus attenuated.

It may also be desirable when tapping or following tapping to be able to easily apply the slag remaining in the converter following the tapping to the refractory lining of the converter. Due to this applied slag layer, a corrosive attack of a metal melt on the lining of the converter can be reduced. The process of applying the slag to the converter is also referred to as “maintaining” the converter. The known methods for maintaining the converter on the one hand include what is known as “slag washing”, in which the slag is distributed over the tapping and charging side by pivoting the converter. A further maintaining method is what is known as “slag splashing”, in which the slag is mechanically sprayed with the aid of a nitrogen gas flow of a lance. Lastly, in the case of what is known as “slag foaming”, slag is chemically foamed by adding a carbon carrier. The slag foamed in the case of slag foaming is also referred to as “foamed slag”.

Besides the maintaining of the converter by means of the foamed slag, this also has further advantageous effects. The foamed slag has insulating properties, such that the heat losses from the melt can be attenuated and energy can be saved. Furthermore, components of the metallurgical vessel in which the iron melt is located may be protected by the foamed slag against heat radiation.

In order to generate a foamed slag in an electric arc furnace, carbon blown into the slag is additionally burned by means of oxygen to form carbon monoxide, and the carbon monoxide gas necessary for foaming is provided in this way. In the case of the melting process in an electric arc furnace, a foaming of the slag layer is of importance, since this shields the light arc by means of a volume increase, attenuates radiation losses at the furnace wall, improves the energy transfer to the melt, and thus likewise saves energy.

In order to increase the reactivity of the slag conditioner when introduced into the slag, it would be desirable in principle to provide the mixture so as to be as fine-grained as possible and to introduce it into the slag in this state. It is therefore known to provide slag conditioners in the form of a fine dust, such that they have a high reactivity on account of their high specific surface area. However, a disadvantage of such slag conditioners provided in the form of a fine dust is in particular the complicated handling of such a slag conditioner in the form of a fine dust. It is therefore known from the prior art to compact a slag conditioner in the form of a fine dust firstly into pellets and to introduce said conditioner in this form into the slag. However, a disadvantage of such slag conditioners pressed into pellets may be that they disintegrate in the slag only with a time delay, and the reactivity of the slag conditioner is thus reduced.

On this basis, one object of the invention is to provide a slag conditioner by means of which the basicity and the MgO content of the slag can be quickly increased in order to be able to reduce the attack of the slag on the refractory lining of the metallurgical vessel in which the metal melt is located with the slag located thereon.

A further object of the invention lies in providing a slag conditioner that has a high reactivity and at the same time can be easily handled.

A further object of the invention lies in providing a slag conditioner by means of which a foaming of the slag can be achieved.

Lastly, a further object of the invention lies in providing a slag conditioner by means of which an increase of the iron output of the primary metallurgical process can be achieved.

In order to achieve these objects, a mixture or a slag conditioner to be introduced into the slag located on a metal melt in iron and steel metallurgy is provided in accordance with the invention, said mixture or slag conditioner comprising the following components in the following mass proportions: raw dolomite in a range from 10 to 90 mass %;

one or more further components comprising at least the following constituents: MgO and carbon, in a range from 90 to 10 mass %.

The mixture according to the invention or the slag conditioner according to the invention is suitable for introduction into slags on metal melts in any metallurgical vessel, but in particular for slags in converters, electric arc furnaces, and ladles.

All of the values specified herein in % are values in mass %, in each case in relation to the total mass of the mixture according to the invention, unless specified otherwise in individual cases.

The proportion of raw dolomite in the mixture according to the invention, which in accordance with the invention lies in a range from 10 to 90 mass %, in particular performs two key tasks in accordance with the invention. On the one hand the raw dolomite is heated suddenly upon contact with the hot slag, whereby a calcination of the raw dolomite is initiated. This calcination leads to a foaming of the slag, such that, when the mixture according to the invention is introduced into the slag, a foamed slag is formed. On the other hand, however, the calcination of the raw dolomite upon introduction of the mixture into the slag also causes the mixture, insofar as this is introduced in the form of pellets into the slag, to spontaneously disintegrate, the mixture thus forming a large specific surface area with a high reactivity. The mixture according to the invention thus makes it possible for said mixture to be provided simultaneously both in the form of pellets, i.e. with good handling, and with a high reactivity.

Due to the proportions of MgO and CaO formed from the raw dolomite following calcination thereof, the basicity of the slag is increased. Furthermore, due to the proportion of MgO formed following calcination of the raw dolomite, an MgO saturation of the slag is achieved, such that the corrosive attack of the slag on the refractory lining of the metallurgical vessel holding the metal melt is reduced. Here, a further advantage of the mixture according to the invention in particular is that this MgO saturation can be achieved particularly quickly on account of the high reactivity of the mixture.

Besides raw dolomite, the mixture according to the invention also comprises one or more further components comprising MgO and carbon.

The proportion of carbon in the further component serves to intensify the foaming of the slag when the mixture is introduced into said slag. Here, when the mixture is added to the slag, the carbon reacts with oxygen located in the slag to form carbon oxides, in particular to form carbon monoxide (CO) and carbon dioxide (CO2). When the mixture is introduced into the slag, the carbon in the further component oxidises immediately and fiercely with oxygen proportions of the slag, such that the slag foams up spontaneously when the mixture is introduced into the slag.

Both due to the calcination of the raw dolomite upon introduction into a slag and due to the reaction of the carbon proportion of the mixture upon introduction into the slag, a foaming of the slag is therefore provoked, whereby the slag, as in the case of slag foaming, increases in height and covers the refractory lining of the metallurgical vessel. In an electric arc furnace the radiation of the electric arcs is shielded partially or completely with respect to the furnace wall as a result of the increased volume of the foamed slag. Due to the increased content of MgO, the slag at the same time obtains the necessary viscosity in order to also remain adhered to the wall during and after the foaming.

If the mixture comes into direct contact with the metal melt, for example because it arrives through a rinser at an opening in the slag layer, the carbon in the mixture may react directly with oxygen of the metal melt and may remove oxygen from the metal melt. This oxygen removed from the metal melt must not later be removed to a greater extent from the metal melt in additional steps by deoxidising agents, for example aluminium.

At least some of the oxygen with which the carbon from the mixture according to the invention introduced into the slag reacts originates from iron oxides in the slag that are reduced by the carbon to form metallic iron. The output of recovered iron in the overall process is thus increased.

Due to the proportion of carbon in the mixture, a foaming of the slag can thus be achieved on the one hand. In addition, the output of recovered iron in the overall process can be increased.

The proportion of MgO of the further components serves in particular to be able to selectively adjust the basicity and the MgO saturation of the slag depending on the other components of the mixture.

The component of the mixture in the form of raw dolomite is raw, i.e. natural, substantially untreated, in particular uncalcined dolomite. Raw dolomite, as is known, is a rock containing the primary mineral dolomite (CaMg(CO₃)₂ or CaCO₃.MgCO₃). Raw dolomite regularly contains dolomite to an extent of at least 90 mass %, in relation to the raw dolomite. In addition, natural impurities may of course be present, for example in the form of Fe₂O₃, SiO₂ or Al₂O₃. In accordance with the invention a raw dolomite having a proportion of dolomite of at least 90 mass %, in relation to the raw dolomite, in particular having at least 91, 92, 93, 94, 95 or 96% dolomite, is preferably used.

Raw dolomite may be present in the mixture according to the invention in a proportion in a range from 10 to 90 mass %, i.e. for example also in a proportion of at least 10, 11, 12, 13, 14 or 15 mass %. By way of example raw dolomite may be present in the mixture according to the invention in a proportion of at most 90, 80, 70, 60, 55, 50, 45, 40, 35, 30 or 25 mass %.

The at least one further component comprised by the mixture besides raw dolomite may be, for example, one or more of the following components: caustic magnesite, magnesia-carbon, sintered magnesia, fused magnesia, graphite, coke, or one or more carbides, for example aluminium carbide. If one or more of the aforementioned components comprising MgO, but no carbon is present in the mixture according to the invention (such as caustic magnesite, sintered magnesia or fused magnesia), at least one further component that comprises carbon is also present (for example at least one of the components magnesia-carbon, graphite, coke or at least one carbide). Accordingly, if one or more of the aforementioned components comprising carbon, but no MgO (such as graphite, coke or at least one carbide) is/are present, the mixture also contains at least one further of the aforementioned components comprising MgO (such as caustic magnesite, sintered magnesia or fused magnesia).

In accordance with a preferred embodiment the further components are present in the form of magnesia-carbon and optionally at least in the form of one of the further aforementioned components.

In a particularly preferred embodiment the further component is present in the form of magnesia-carbon and in the form of caustic magnesite.

Magnesia-carbon constitutes what are known as magnesia-carbon products, which are also referred to as magnesia-carbon bricks or MgO—C bricks and are characterised by grains formed from magnesia (MgO) and interconnected via a carbon bond. In accordance with a particularly preferred embodiment magnesia-carbon is present in the form of used magnesia-carbon products, i.e. what is known as broken-off magnesia-carbon. Such broken-off magnesia-carbon constitutes magnesia-carbon products that have been used in the steel industry, in particular as inner linings of basic oxygen furnace converters, in electric arc furnaces, or in ladles. In this respect, correspondingly recycled magnesia-carbon products can be present partially, largely or exclusively as magnesia-carbon in the mixture according to the invention. In this respect, the invention also relates to the use of recycled magnesia-carbon products as raw material for the mixture according to the invention and the use of such recycled magnesia-carbon products as a component of a slag conditioner according to the invention. It has been found in accordance with the invention that the use of magnesia-carbon as carbon carrier in the mixture according to the invention may be particularly advantageous, in particular insofar as magnesia-carbon, in particular if present in broken-off form, can be approximated to the composition of the refractory lining of a metallurgical melting vessel, and therefore, when forming a foamed slag, may contribute in a particularly effective manner to the maintaining of the lining by the foamed slag.

Caustic magnesia is caustically burned magnesite, which is sometimes also referred to as caustic magnesite or caustic magnesia. Caustic magnesite is obtained, as is known, by caustic firing of magnesite (MgCO₃), i.e. by firing magnesite at low temperatures. An advantage of the use of caustic magnesite in the mixture according to the invention is in particular the fact that it is highly reactive, which is why MgO, when introduced into a slag, can be released particularly quickly and the increase of the basicity of the slag and also the MgO saturation thereof therefore can be achieved particularly quickly.

It has now been found in accordance with the invention that the mixture according to the invention has particularly advantageous properties, in particular with regard to its reactivity and efficacy as an agent for foaming the slag, if the mixture comprises the further components in the form of magnesia-carbon and caustic magnesite in the following mass proportions: magnesia-carbon: 10 to 50 mass %; caustic magnesite: 10 to 50 mass %. Here, in accordance with the invention, it is also possible that only one of the aforementioned components is present in the mixture in the aforementioned mass proportions.

Magnesia-carbon may be present in the mixture for example in proportions of at least 10, 15, 20, 25, 30 or 35 mass %. Furthermore, magnesia-carbon may be present in the mixture for example in proportions of at most 45 or 40 mass %.

Caustic magnesite may be present in the mixture for example in proportions of at least 10, 15, 20, 25, 30 or 35 mass %. Furthermore, caustic magnesite may be present in the mixture for example in proportions of at most 50 or 45 mass %.

The mixture is preferably present in a relatively low grain size, for example in a grain size less than 5 mm to an extent of at least 50 mass %, 60 mass %, 70 mass %, 80 mass %, 90 mass %, or also 100 mass %.

By way of example, raw dolomite may be present in a grain size less than 5 mm to an extent of at least 80 mass %, i.e. for example also to an extent of at least 90 mass % or 100 mass %.

The further components, in particular if present in the form of magnesia-carbon and caustic magnesite, may preferably be present in a grain size less than 1 mm to an extent of at least 80 mass %, i.e. for example also at least 90 mass % or 100 mass %.

Since the mixture according to the invention has this very small mean grain size, a particularly good and uniform distribution and particularly also a quick dissolution of the mixture in the slag can be achieved.

In order to be able to achieve good handling of the mixture according to the invention in spite of this small grain size of the mixture, the mixture may be provided in compacted or pressed form, for example in the form of pellets. In order to provide the mixture in the form of pellets, it can be provided that a mixture according to the invention, which in particular may have the previously described grain size distribution, can be pressed into pellets without addition of additives.

By way of example, these pellets may have an almond-shaped, rod-shaped or spherical form, for example with a maximum length of for example 50 mm, 40 mm, or 30 mm. The pellets may also have a minimum diameter for example of 5, 10, 15, 20, or 25 mm. Pellets with a corresponding size can be easily handled, but at the same time are still small enough that they quickly disintegrate in a slag following addition thereto, and the advantages of the small grain size distribution according to the invention can materialise there quickly.

As discussed above, the spontaneous calcination of the raw dolomite upon introduction of the mixture into the slag causes the pellets, upon introduction into the slag, to disintegrate spontaneously and thus a small grain size or high fineness of grain of the mixture with a high specific surface area and accompanying high reactivity develop immediately.

The mixture may contain a proportion of magnesium carbonate (MgCO₃) for example in a range from 5 to 30 mass %, i.e. for example also a proportion of at least 6, 7, 8 or 9 mass % and for example a proportion of at most 25, 20 or 15 mass %. The proportion of magnesium carbonate in the mixture may be present predominantly or completely as a constituent of the raw dolomite.

The proportion of calcium carbonate (CaCO3) in the mixture for example may lie in a range from 5 to 35 mass %, i.e. for example also at a level of at least 6, 7, 8, 9 or 10 mass % and for example also at a level of at most 30, 25, 20 or 15 mass %. The proportion of calcium carbonate in the mixture may be present predominantly or completely as a constituent of the raw dolomite.

The proportion of magnesia (MgO) in the mixture for example may lie in a range from 20 to 60 mass %, i.e. for example also at a level of at least 25, 30, 35, 40 or 45 mass % and for example also at a level of at most 55 or 50 mass %. The magnesia for example may be present in particular in the components constituted by magnesia-carbon and caustic magnesite.

Carbon may be present in the mixture according to the invention for example in a proportion in a range from 3 to 15 mass %, i.e. for example also in a proportion of at least 4, 5 or 6 mass %, and for example also in a proportion of at most 14, 13, 12, 11 or 10 mass %. Carbon may be present in the mixture predominantly or completely in the component in the form of magnesia-carbon.

Calcium oxide (CaO), which for example may be introduced into the mixture as a minor constituent of caustic magnesite, may be present in the mixture for example in proportions in a range from 0 to 40 mass %, i.e. for example also in proportions of at least 5, 10 or 15 mass % and for example also in proportions of at most 35, 30, 25 or 20 mass %. Further substances, for example SiO₂, Fe₂O₃, Al₂O₃, H₂O or P₂O₅, may be introduced into the mixture as impurities via the components of the mixture. Here, these materials may be present in the mixture in the following proportions for example, wherein the proportions of these substances may be present in the mixture individually or in any combination in the following proportions:

SiO₂: 0 to 10 mass %, i.e. for example also in proportions of at least 0.5 or 1 mass % and for example in proportions of at most 9, 8, 7, 6, 5, 4, 3 or 2 mass %;

Fe₂O₃: 0 to 10 mass %, i.e. for example also in proportions of at least 0.5 or 1 mass % and for example in proportions of at most 9, 8, 7, 6, 5, 4, 3 or 2 mass %;

Al₂O₃: 0 to 5 mass %, i.e. for example also in proportions of at least 0.5 or 1 or 1.5 or 2 mass % and for example in proportions of at most 4, 3 or 2.5 mass %;

H₂O: 0 to 3 mass %, i.e. for example also in proportions of at least 0.5 or 1 mass % and for example in proportions of most 2 or 1 mass %;

P₂O₅: 0 to 0.5 mass %.

It has been found in accordance with the invention that the mixture may react very sensitively to further components and substances. In this respect, it may be that the mixture according to the invention, besides components in the form of raw magnesite, magnesia-carbon and caustic magnesite, also contains proportions of further components in a proportion less than 10 mass %, i.e. for example also in a proportion less than 9, 8, 7, 6, 5, 4, 3, 2 or 1 mass %. It may be that the mixture, besides the aforementioned substances MgCO3, CaCO₃, MgO, C, CaO, SiO₂, Fe₂O₃, Al₂O₃, H₂O, P₂O₅, in particular in the aforementioned mass proportions, also contains further substances only in a proportion less than 10 mass %, i.e. for example also in a proportion less than 9, 8, 7, 6, 5, 4, 3, 2 or 1 mass %.

In order to provide the mixture according to the invention in the form of pellets in accordance with the invention, the mixture may be made up with a binder before being pressed into pellets. By way of example, a sulphate-based binder may be provided, for example an Epsom salt solution. The concentration of Epsom salt in the Epsom salt solution may lie for example in a range from 0.1 to 0.3 mass %. The mixture may be made up with a proportion of binder in a range from 5 to 15 mass %, in relation to the mixture without the binder.

In order to improve the green strength of the pellets, it can be provided that the mixture may also comprise, in addition, one or more temporary binders, for example one or more of the following temporary binders: glucose, starch, one or more silica-based binders, or one or more phosphate-based binders.

The invention also relates to a method for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy, said method comprising the following steps:

providing a mixture according to the invention; introducing the mixture into the slag located on the metal melt in a metallurgical vessel.

As described herein, the mixture thus can also be provided for example in compacted or pressed form, for example in the form of pellets.

Here, the mixture may be mixed with one or more of the binders described herein and then pressed into pellets, in particular having the dimensions described herein.

The mixture provided, for example in the form of pellets, is introduced onto the slag and sinks thereinto, such that it can develop its effect according to the invention there.

The mixture according to the invention is suitable in principle as a slag conditioner for slags on a metal melt in any metallurgical vessel, for example for metal melts in converters, electric arc furnaces, or ladles. The mixture according to the invention is particularly preferably used as a slag conditioner for slags on metal melts located in a metallurgical vessel with an alkaline lining, i.e. in particular with a lining based on at least one of the following materials: magnesia, magnesia-carbon, doloma or doloma-carbon.

The invention also relates to the use of a mixture according to the invention described herein for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy

Here, the use may be implemented as disclosed herein.

All of the features of the invention disclosed herein may be combined with one another arbitrarily, individually or in combination.

The invention will be explained in greater detail on the basis of the following practical example.

In the practical example a mixture containing the following components according to Table 1 was firstly provided.

TABLE 1 Component mass proportions [%] raw dolomite 22 magnesia-carbon 40 caustic magnesite 38

The used raw dolomite contained a proportion of dolomite of more than 95 mass %, in relation to the raw dolomite. In particular, Al₂O₃, Fe₂O₃ and SiO2 were present as minor constituents.

The component in the form of magnesia-carbon was broken-off material in the form of recycled magnesia-carbon products. Besides the primary constituent MgO, this component also contained a proportion of carbon of 28 mass %, in relation to the total mass of the components.

The raw dolomite was present in a grain size less than 5 mm, and the components magnesia-carbon and caustic magnesite were present in a grain size less than 1 mm.

The mixture was mixed with 10% binder in the form of Epsom salt solution, in relation to the mass of the mixture without the binder, and was pressed into almond-shaped pellets having a thickness of approximately 15 mm and a length of approximately 30 mm.

The pellets were then dried at approximately 300° Celsius and, once cool, were provided in this form as slag conditioner. 

1. A mixture to be introduced into the slag located on a metal melt in iron and steel metallurgy, said mixture comprising the following components in the following mass proportions: raw dolomite in a range from 10 to 90 mass %; one or more further components comprising at least the following constituents: MgO and carbon, in a range from 90 to 10 mass %.
 2. The mixture according to claim 1 with a proportion of raw dolomite in a range from 10 to 55 mass %.
 3. The mixture according to claim 1, in which the further components are present in the form of magnesia-carbon and caustic magnesite.
 4. The mixture according to claim 1, in which the mixture comprises further components in the form of magnesia-carbon and caustic magnesite in the following mass proportions: magnesia-carbon: 10 to 50 mass %; caustic magnesite: 10 to 50 mass %.
 5. The mixture according to claim 1, which mixture is present in the form of pellets.
 6. A method for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy, said method comprising the following steps: providing a mixture, said mixture comprising the following components in the following mass proportions: raw dolomite in a range from 10 to 90 mass %; one or more further components comprising at least the following constituents: MgO and carbon, in a range from 90 to 10 mass %; and introducing the mixture into the slag located on the metal melt in the metallurgical vessel.
 7. A method for using a mixture for conditioning a slag located on a metal melt in a metallurgical vessel in iron and steel metallurgy, said mixture comprising the following components in the following mass proportions: raw dolomite in a range from 10 to 90 mass %; one or more further components comprising at least the following constituents: MgO and carbon, in a range from 90 to 10 mass %. 